Optically variable element and the use thereof

ABSTRACT

The invention relates to an optically variable element which at least in surface portions has an interface embedded between two layers and which forms an optically effective structure, that interface having a free-form surface which appears three-dimensionally for a viewer. To emphasise that free-form surface the invention provides that the free-form surface is formed by a partial region of the interface, which is of a lens-like configuration and which produces a magnification, reduction or distortion effect. The invention also provides the use of such optically variable elements as a security element to prevent forgery of value-bearing documents or for articles to be safeguarded, in particular as part of the decorative layer arrangement of a transfer or laminating film.

The invention concerns an optically variable element which at least insurface portions has an interface which is preferably embedded betweentwo layers of a layer composite and which forms an optically effectivestructure which spatially projects and/or is set back with respect to a(notional) reference surface, wherein the optically effective structurehas at least one free-form surface appearing three-dimensionally for aviewer in the form of an alphanumeric character, a geometrical figure oranother object.

Optically variable elements of the above-described kind are used forexample as security elements for authenticating or identifyingvalue-bearing documents, for example banknotes, cheques, etc, identitycards and passes, credit cards or other articles to be safeguarded. Suchoptically variable elements are also already used for decorativepurposes, in which respect the boundary between use as a securityelement and use as a decorative element is frequently fluid. In thatrespect a particularly frequent requirement is that security elementsalso have a certain decorative effect, which applies for example whenthe situation involves guaranteeing the authenticity of certainarticles, for example cigarettes, valuable cosmetic preparations and soforth, by corresponding elements.

For use as a security or decorative element, the known opticallyvariable elements are generally applied to the corresponding substratein the form of transfer films, in particular hot stamping films, or inthe form of laminating films, in which case the interface forming theoptically effective structure is then provided between two correspondinglacquer layers. In the case of transfer films those lacquer layers arepart of the decorative layer arrangement which can be transferred fromthe carrier film on to the substrate, wherein instead of a lacquer layerit is also possible to provide an adhesive layer or the lacquer layermay have adhesive properties. In the case of laminating films theinterfaces are in principle produced in the same way. The differencebetween laminating and transfer films however is that, in the case oflaminating films, the lacquer and possibly adhesive layers serving asthe decorative element remain on the carrier film when the laminatingfilm is applied to a substrate. Finally it is also conceivable forpackaging or decorative films to be basically like laminating films, butfor those films, for example for packaging purposes, to be used as suchwithout being laminated on to a substrate.

In this connection it is also already known for three-dimensionaleffects to be produced by way of suitable structuring of the interfacebetween two layers, in particular lacquer layers, or in relation to air.For example cheque and credit cards are known, in which certain objectsappear in different positions or perspectives in dependence on theviewing angle, or the impression is given to the viewer as though thecorresponding object were standing three-dimensionally out of thesurface of the carrier for the optically variable element.

Hitherto those three-dimensional effects were generally producedholographically, in which respect that procedure has on the one hand thedisadvantage that a comparatively high level of apparatus expenditure isinvolved in production of the masters required for replication incorresponding layers. In addition holographically produced structuresalso suffer from serious optical disadvantages. In particular theirshine is frequently defective. In addition, there is generally nopossible way of increasing the attractiveness of a correspondinglyoptically variable element by achieving certain colour effects.

Therefore the object of the present invention is to propose an opticallyvariable element which can easily be produced with the most widelyvarying processes known for the production of optically effectivestructures, which exhibits hitherto unknown effects from the point ofview of the viewer and which in addition offers a designer a largernumber of possible variations in respect of design configuration.

In an optically variable element of the general kind set forth, inaccordance with the invention that object is attained in that thefree-form surface is formed by a partial region of the interface, whichpartial region is of a lens-like configuration and produces amagnification, reduction or distortion effect and forms a free-formelement.

While therefore hitherto the three-dimensional free-form surfaces, forexample birds, letter or character combinations, pictures of people,mountains and so forth only appear in such a way as though either theywould change their position upon a change in the viewing angle or theyappear to float over the surface of the substrate, completely differentoptical effects are proposed in accordance with the invention, namelythe optically variable element is of such a nature that the regionforming the free-form surface, for example letters, digits but also anyother objects, logos and so forth appears in such a way as though itwere curved forwardly with respect to the surface of the substrate orwould be set back, that is to say as though a curved surface werepresent in the region of the free-form surface. From the point of viewof the viewer, that gives rise to a completely novel, hitherto unknowneffect for the optically effective structure, namely that of a certainspatial depth, wherein in addition, with a suitable configuration andarrangement of the lens-like partial region of the interface,particularly characteristic optical effects can be achieved, whichgreatly enhance the recognition value and thus the identification effectof corresponding optically variable elements.

If the dimensions of the free-form surface are very small, that is tosay if for example this involves an alphanumeric character with a verysmall line thickness, the effect according to the invention for anoptically variable element can already be achieved by the free-formsurface being of a configuration like a refractive lens structure. It isto be borne in mind however that the layers, between which the interfaceforming the optically effective structure is arranged, are usuallylacquer layers which normally can only be of a very limited thickness.In order to be able to achieve the desired effect according to theinvention, even when comparatively thin lacquer or adhesive layers areinvolved, it is desirable if the free-form surface is in the form of adiffractive free-form element with a grating structure whose gratingdepth is at most 10 μm and which has grating lines substantiallyfollowing the contour lines of the free-form surface, wherein thespacing of the grating lines from the central region of the free-formsurface towards the edge thereof continuously changes, that is to sayeither decreases or increases.

In a configuration of the optically variable element according to theinvention the grating structure of the free-form element can be of sucha configuration that the respective one flanks of the grating groovesextend in mutually parallel relationship and in approximately parallelrelationship with a normal to the (notional) reference surface, whilethe angle of the respective other flanks of the grating grooves relativeto the normal to the reference surface changes in a directiontransversely with respect to the grating lines substantiallycontinuously from one grating groove to another grating groove, whereinit will be assumed self-evident that the grating grooves are of areducing cross-section.

The production of such grating structures is preferably effected bymeans of the so-called ‘direct writing’ process by means of laser orelectron beam lithography machines, the use of which makes it possibleto produce quite specific grating structures, that is to say, toactually accurately produce the desired optical effect for the free-formelement.

It will be noted however that it is also possible for theabove-mentioned grating structure with grating grooves whose flanks arearranged at an angle relative to each other to be produced in adifferent manner than by ‘direct writing’, more specifically when theflanks of the grating grooves, which extend at an angle to the normal tothe reference surface, are of a stepped configuration, in which case theflanks—extending at an angle relative to the normal to the referencesurface—are approximated in their optical effect by the surfaces formingthe steps. When the flanks of the grating grooves are of such aconfiguration it is possible for example also to operate by means ofmasks, in which case the fineness of the stepped resolution of the(inclined) flanks depends on the number of masks used, that is to saythe desired steps. In that respect, division of the corresponding flanksinto four or eight steps is already sufficient for a large number ofsituations of use. When high quality demands are involved however it isalso possible to provide for example sixty four steps, for theproduction of which a corresponding number of exposure operations isnecessary, using different masks.

Production of the grating structure of the free-form element, which isvery simple under some circumstances, can be achieved when the gratingstructure is a binary structure which has substantially rectangulargrating grooves and grating lands, wherein preferably the configurationis such that the depth of the grating grooves of the grating structureof the free-form elements is approximately equal over the entirefree-form surface, that is to say the change in the ‘refractive power’(diffraction of the light into different directions) is only achieved bythe width of the grating grooves and/or grating lands being suitablyvaried.

A particularity of the diffractive free-form elements formed by gratingstructures, in accordance with the invention, is that such diffractivelens structures—unlike refractive lenses—produce a different visualimpression, in dependence on the light wavelength respectively used forillumination or viewing of the object, whereby once again it is possibleto achieve particular design or security effects.

A further possible way of producing three-dimensionally appearingfree-form surfaces according to the invention provides that thefree-form surface is formed by a holographically produced free-formelement, in which respect holographically produced lenses do howeversuffer from certain disadvantages in comparison with diffractive lenselements. For example, lens elements can be holographically produced atreasonable expense only if the configuration of the free-form surface iscomparatively simple. In addition, because of their sinusoidalstructure, holographically produced lenses do not appear too brilliantand frequently suffer from non-homogeneities, whereby the visualappearance which is to be produced by the lens can be adverselyaffected. In addition certain colour effects cannot be achieved with thedesired high degree of freedom in terms of design configuration, withholographically produced lens elements.

It is basically conceivable for an optically variable element whichessentially has a free-form surface designed according to the inventionto be used as a security or decorative element. Advantageously howeverthe free-form surface is part of an optically effective overallstructure arrangement which, besides the free-form element, includespartial regions with optically variable elements which for the viewerproduce different optical effects. For example a free-form element canbe combined with the usual structures having an optical-diffractioneffect, as are known for example, to produce motion effects, flips,changes between two different representations, and so forth. It will beappreciated that it is also possible to combine in one opticallyvariable element a plurality of free-form elements, for example to makeup a word or a number from letters or digits each forming its ownfree-form element, whereby then that gives the impression as though theword or the number were three-dimensionally emphasised in relation tothe rest of the optically variable element. Attractive effects are alsoafforded if a plurality of free-form elements are so-to-speakinterleaved with each other so that then, when different illumination orviewing directions are involved, the respective different free-formelements are visible. In principle there is here such a large number ofpossible combinations, for example including with matt effects, specularsurfaces and so forth, that a more detailed discussion is not to be setforth at this juncture.

A possibility of particular interest is that of combining the opticallyeffective structure with a thin-film arrangement completely or inregion-wise manner, whereby it is possible to achieve specific colourchanges, in dependence on the viewing angle. Further special effects canbe achieved by the use of semiconductor layers.

It is further provided according to the invention that the interfaceforming the optically effective structure is provided at leastregion-wise with a reflection-enhancing coating which, if observation ofthe corresponding effect is to occur actually only with top light, thatis to say in a reflection mode, is desirably formed by a metal layer. Itwill be noted that it is also possible, instead of the metal layer asthe reflection-enhancing coating, to provide a dielectric layer having arefractive index which is suitably different with respect to theadjoining layers, or however also a suitably configured multi-layerarrangement or semiconductor coating.

It is possible to emphasise the free-form element in accordance with theinvention in a simple manner if the reflection-enhancing coating isprovided in register relationship with the at least one free-formelement, wherein the register relationship can either be such that thereflection-enhancing coating is present only in the region of thefree-form element, or however it is such that it is precisely in theregion of the free-form element that there is no reflection-enhancingcoating, but it is provided only in the region of the optically variableelement, that surrounds the free-form element. That configuration can behighly advantageous for example when there are provided around thefree-form element elements or structures which only produce verymarkedly discernible effects in reflection, for example motion effects,image changes and so forth.

The register relationship in respect of the reflection-enhancingcoating, when a metal layer serves as the coating, can be easilyproduced by the per se known processes or region-wise demetallisation ofthe interface layer.

As can be seen from the foregoing description the optically variableelement according to the invention can be used in different ways and forthe most widely varying purposes. However the use of an opticallyvariable element according to the invention as a security element inrelation to forgery of value-bearing documents or for articles to besafeguarded is particularly advantageous, in particular also for thereason that the lens-like free-form elements provided according to theinvention afford the possibility of introducing into the securityelement additional identification or safeguard features which differfrom the features known hitherto for security elements in a novel mannerand thus in a striking fashion from the point of view of the user of thecorresponding document or the article to be safeguarded.

The use of an optically variable element according to the invention as asecurity element is advantageously effected in that the opticallyvariable element is incorporated into the decorative layer arrangement,which can be transferred on to a substrate, of a transfer film, inparticular a hot stamping film, or into the decorative layer arrangementof a laminating film, because that simplifies either transfer on to asubstrate or the production of labels and so forth in a designconfiguration according to the invention.

Further features, details and advantages of the invention will beapparent from the description hereinafter with reference to the drawingin which:

FIG. 1 a diagrammatically shows a section through a refractive lens,

FIG. 1 b shows a section through a corresponding diffractive lens withgrating grooves of approximately triangular cross-section,

FIG. 1 c shows a diffractive lens similar to FIG. 1 b with a diffractivebinary structure,

FIG. 2 a shows a perspective view of a wave-like free-form surface,

FIG. 2 b shows a plan view in highly diagrammatic and rough form showingthe free-form surface of FIG. 2 a in the form of a diffractive free-formelement with a grating structure as shown in FIG. 1 b,

FIG. 2 c shows a plan view corresponding to FIG. 2 b but in the case ofa free-form element with a diffractive binary structure as shown in FIG.1 c,

FIG. 3 a is a perspective view of a free-form surface in the form of adrop as a refractive configuration,

FIG. 3 b is a graph representation of the configuration of the interfaceof the drop-shaped free-form surface of FIG. 3 a,

FIGS. 4 a and 4 b are views corresponding to FIGS. 3 a and 3 b but withthe drop-shaped free-form surface in the form of a diffractive free-formelement with grating grooves of approximately triangular cross-section,

FIGS. 5 a and 5 b are views corresponding to FIGS. 3 a, 3 b and

FIGS. 4 a, 4 b respectively but with the free-form element in the formof a diffractive binary structure,

FIGS. 6 a and 6 b are illustrations corresponding to FIGS. 3 a and 3 bfor an annular free-form surface,

FIGS. 7 a, 7 b and 7 c are illustrations in respect of the annularfree-form surface corresponding to FIGS. 4 a, 4 b and 5 b of thedrop-shaped free-form surface,

FIGS. 8 a and 8 b are illustrations of an L-shaped free-form surfacecorresponding to FIGS. 3 a, 3 b and FIGS. 5 a, 5 b respectively (dropand ring),

FIGS. 9 a, 9 b and 9 c are illustrations corresponding to FIGS. 7 a, 7 band 7 c for the L-shaped free-form surface, and

FIG. 10 is a plan view of an optically variable element with a weavepattern forming the free-form surface.

The highly diagrammatic and relatively rough views in FIGS. 1 a to 1 ceach show the partial region, which has a lens-like action, of anoptically variable element according to the invention wherein formedbetween two layers 1, 2 which are generally lacquer layers is aninterface 3 which is generally provided with a reflection-enhancingcoating (not additionally shown in the drawing), for example ametallisation in the form of a vapour-deposited metal layer. In thatrespect, shown on the x-axis of FIGS. 1 a to 1 c is the dimension of thecorresponding lens element in the respective direction, wherein theunits of FIGS. 1 a to 1 c involve any assumed units as the precise sizeor the precise diameter of the lens elements is not an importantconsideration. In general terms the corresponding dimensions of the lenselements or the free-form elements formed by the lens elements howeverare between 0.15 and 300 mm, preferably between 3 and 50 mm.

Plotted on the y-axis in FIGS. 1 a to 1 c in each case is the thicknessor the height respectively of the corresponding layers 1, 2 and therefractive surface or structure formed by the interface 3 respectively,the specified values being the phase difference in radians. When using aspecific wavelength (for example 550 nm for the maximum sensitivity ofthe human eye), the actual geometrical depth can be calculated from thatphase difference in known manner (also having regard to the respectiverefractive index).

If FIG. 1 a is compared to FIGS. 1 b and 1 c, it can be see that thethickness of the optically variable element of FIG. 1 a must be at leastten times as large as the thickness of the layer arrangement forming theoptically variable element in FIG. 1 b and even twenty times as great asthe thickness of the layer arrangement in FIG. 1 c. In this case, thefact that the layer arrangements of FIGS. 1 b and 1 c which form theoptically variable element can be substantially thinner than that inFIG. 1 a is due to the smaller overall height h of the structure whichis determined by the interface 3 and which produces the lens effect andwhich extends only over a height which, when converted (for a systemn=1.5/n 32 1 in the transmission mode) in FIGS. 1 b correspondsapproximately to double the wavelength and in FIG. 1 c even onlyapproximately the single wavelength. At any event the height h, that isto say the grating depth, is no greater than 10 μm, in the diffractivelens elements of FIGS. 1 b and 1 c.

As already mentioned the layers 1 and 2 are generally lacquer layers ofsuitable composition, wherein at least the lacquer layer which istowards the viewer (in the present case generally the layer 1) must besubstantially transparent, although it will be noted that there is alsothe possibility of the lacquer layers being coloured while substantiallypreserving transparency. For certain situations of use one of the layers1, 2 may also be an adhesive layer or at least a lacquer layer havingsuitable adhesive properties.

If the interface 3 is provided with a metallisation or another, stronglyreflecting layer, the layer 2 can admittedly also be transparent but itmay also be translucent or opaque. If in contrast the optically variableelement according to the invention is to be used in the transmissionmode, for example for covering over a visible feature on a substrate,the layer 2 must also be transparent. In that case the interface is notprovided with a—generally opaque—metallisation. Instead, the refractiveindex of the two transparent layers 1 and 2 will be selected to bedifferent in such a way (the difference in the refractive indices shouldpreferably be at least 0.2) that, in spite of the use of two transparentlayers, the optical effect produced by the interface 3 becomessufficiently clearly visible.

If difficulties arise in that respect in implementing a sufficientlygreat difference in the refractive index of the layers, it would also bepossible in accordance with the invention for the grating grooves of thefree-form elements to be partially or substantially filled with atransparent material which has a sufficiently greatly differingrefractive index before the continuous layer which faces towards theviewer is applied.

The master necessary for production of the lens element shown in FIG. 1a in a—basically known—replication process can be produced by mechanicalprecision removal processes with comparative ease in regard to thedimensions which are substantially larger in comparison with thestructures of the lens elements of FIGS. 1 b and 1 c.

The diffractive grating structure of the lens element of FIG. 1 b isusually produced in a so-called ‘direct writing process’, that is to saya process in which the material is removed in accordance with thedesired profile by means of a laser or a photoresist is exposed inaccordance with the desired profile by means of a laser or an electronbeam lithography device and then the desired profile or the negativeprofile thereof is obtained by development of the photoresist. Thatprocedure affords the advantage that very different grating structuresand in particular grating cross-sections can be produced, for exampleincluding so-called blaze gratings for specific situations of use, inwhich respect it can particularly be provided that the angle a betweenthe flanks 4 of the grating grooves 5, which flanks extend inclinedly inFIG. 1 b, and a normal S on a notional reference surface, extendingparallel to the x-axis, of the grating structure forming the lenselement changes continuously from the paraboloidal central region 6 ofthe interface 3 forming the lens element in an outward direction—as isclearly apparent from FIG. 1 b—and more specifically in such a fashionthat, in the illustrated embodiment, the flanks 7 of the grating grooves5, which are approximately parallel to the normal S, representso-to-speak only discontinuities in an otherwise substantially steadylens profile which is formed by the successive inclined flanks 4 of thegrating grooves 5 and the central paraboloidal portion 6 of theinterface 3.

Lens structures of that kind and the manner of calculating same arebasically described in the relevant literature in the art, and for thatreason they will not be discussed in greater detail here.

In this respect mention should also be made of the possibility, in placeof the inclined flanks 4 which are continuous over the height h as shownin FIG. 1 b, of using a stepped arrangement in which the surfacesforming the steps approximate to the flanks 4 in respect of theiroptical effect. Grating structures of that kind can be produced bothusing a so-called direct writing process and also by way of suitablemask procedures, in which respect the number of steps can be varied independence on the desired result. In that case, division into four oreight steps is already sufficient for a large number of situations ofuse. When high quality requirements are involved however it is forexample also possible to provide sixty four steps or a number of stepsat a higher power of 2.

FIG. 1 c diagrammatically shows a lens element formed by a so-calledbinary structure. In this respect the essential characteristic of thebinary structure of FIG. 1 c is that both the grating grooves 8 and alsothe grating lands 9 are each of substantially rectangular cross-section.Binary structures as shown in FIG. 1 c are in that case usually producedusing suitable masks, wherein in this connection the furtherparticularity of the structure of FIG. 1 c is advantageous, namely thatthe grating depth h of the grating structure is uniform over the entirelens element so that production of the associated masters does notinvolve either providing different periods of action for the means forremoving the material nor having to operate with different levels ofintensity of the means acting on the substrate through the correspondingmask.

There is also the possibility of producing suitable lens structures bymeans of per se known holographic processes, in which case that thengives structures of even smaller grating depth and of a substantiallysinusoidal configuration, which however possibly leads to thedisadvantages discussed above.

FIGS. 2 a, 3 a, 6 a and 8 a each show as a somewhat diagrammatic andgreatly enlarged perspective view an illustration of a free-form surfacein the form of a refractive lens element, that is to say a free-formelement, wherein the Figures each only show a perspective view of theinterface 3, which is present between the two layers 1, 2, of thefree-form element, in order to clearly show the principle of theinvention.

In that respect, refractive free-form elements of that kind which aresufficiently optically striking can only be achieved if either thethickness of the layers 1, 2 enclosing the interface 3 between them issufficiently great or if the dimensions of the free-form surfaceparallel to the notional reference surface, for example in FIG. 2 a thebase surface 10, are sufficiently small, because indeed in the case ofrefractive free-form elements the height h of the lens element, as canbe clearly seen from FIG. 1 a, depends directly on the dimensions of thefree-form surface in the direction of the x-axis.

FIG. 3 a shows a drop-shaped free-form element 11, wherein as shown inFIG. 3 a the free-form element 11 forming the drop-shaped free-formsurface is so designed that the free-form surface appears to projectupwardly beyond the otherwise flat interface 3. It will be appreciatedthat it would correspondingly also be possible to produce the impressionas though the drop formed by the free-form element 11 were to projectrearwardly (downwardly) beyond the surrounding interface 3.

FIG. 6 a is a view similar to FIG. 3 a showing an annular refractivefree-form element 12 which for example can symbolise the letter ‘O’ orhowever can also have an only decorative effect.

FIG. 8 a correspondingly shows a perspective view of the interface 3which is produced when the letter ‘L’ is illustrated by a refractivefree-form element 13.

In the same manner as FIGS. 3 a, 6 a and 8 a, FIGS. 3 b, 6 b and 8 beach show—approximately in section perpendicularly to the notionalreference surface—the configuration of the interface 3 in the case ofthe associated free-form elements 11, 12 and 13, wherein the dimensionsof the graph views in FIGS. 3 b, 6 b and 8 b again correspond to FIGS. 1a to 1 c, that is to say any units are shown on the x-axis, while thedeflection perpendicularly to the notional reference surface is shown onthe y-axis in radians. In this case the profile in FIG. 3 b extendsalong the axis of symmetry of the drop-shaped free-form element 11 inFIG. 3 a, more specifically from bottom right in FIG. 3 a to top left,that is to say from the rounded region to the tip of the drop. In regardto FIG. 8 b the profile of the left-hand limb of the ‘L’ is also plottedin each case from bottom right to top left, thereby giving—because ofthe transverse limb of the ‘L’ which branches off at bottom right—theincrease in height in the left-hand region in FIG. 8 b.

It is interesting now to compare the diffractive grating structuresserving as free-form elements to the refractive structures of FIGS. 2 a,3 a, 6 a and 8 a.

FIG. 2 b is a diagrammatic and greatly enlarged plan view of thefree-form surface of FIG. 2 a, and more specifically in a direction ofview approximately perpendicularly on to the reference surface 10, withthe free-form surface being in the form of a diffractive free-formelement with a grating structure having grating lines whichsubstantially follow the contour lines of the free-form surface, whereinthe spacing of the grating lines from the central region of thefree-form element towards the edge thereof continuously changes. Acomparison of FIGS. 2 a and 2 b also shows in this connection that theterm ‘contour lines of the free-form surface’ in accordance with theinvention does not necessarily mean the actual boundary of the free-formsurface. Rather, it is important for the grating structures to extend insuch a way that the spatial configuration of the free-form surface, forexample the differing spacing of the free-form surface of FIG. 2 a fromthe notional reference surface 10, is also suitably taken intoconsideration.

FIG. 2 c is a view also corresponding to the view in FIG. 2 b showing aplan view of the structure of the free-form surface of FIG. 2 a, whenthe lens element is not formed as in FIG. 1 b by a grating structurewith continuously changing grating grooves but instead thereof thegrating structure is a binary structure, as is basically shown in FIG. 1c.

FIGS. 4 a, 7 a and 9 a again basically show plan views corresponding toFIGS. 3 a, 6 a and 8 a, of the drop-shaped free-form element 11, theannular free-form element 12 and the L-shaped free-form element 13respectively, wherein however the free-form element in each case isagain not in the form of a refractive lens but in the form of adiffractive grating structure involving the basic configuration shown inFIG. 1 b.

The sections or height profiles corresponding to FIGS. 3 b, 6 b and 8 bare correspondingly shown in FIGS. 4 b, 7 b and 9 b.

In connection with the drop-shaped free-form surface of FIGS. 3 a and 4a respectively, FIG. 5 a finally also shows a plan view when thefree-form element is in the form of a binary grating, the resultingheightwise profile of the interface 3 being correspondingly shown inFIG. 5 b. In regard to the annular and L-shaped free-form surface, aperspective view of the interface 3 when the free-form element is in theform of a binary structure has not been illustrated herein. Thecorresponding heightwise profiles are however shown in FIGS. 7 c and 9 c(for the annular and L-shaped free-form element respectively).

A corresponding comparison of FIGS. 3 b, 6 b and 8 b with FIGS. 4 b, 7 band 9 b and FIGS. 5 b, 7 c and 9 c respectively again shows the markedreduction in the height of the structures in regard to the transitionfrom a refractive structure (FIGS. 3 b, 6 b, 8 b) to a diffractivecontinuous grating structure (FIGS. 4 b, 7 b and 9 b) and a binarystructure (FIGS. 5 b, 7 c and 9 c) respectively.

Finally FIG. 10 also shows an example of a more complex structure withfree-form surfaces formed by free-form elements. This involves a weaveor grid structure in which the mutually crossing threads 14 and 15respectively are emphasised by virtue of being in the form of free-formelements according to the invention.

The described examples only involve comparatively simple embodimentswhich for example, like FIGS. 3 to 9, each include only one free-formelement. It will be appreciated that it is possible to produce opticallyvariable elements even with complex effects, by a suitable combinationof different free-form elements, in which respect it is also possible inparticular to provide, in addition to the lens-like free-form elementsaccording to the invention, optically active structures, in particulardiffractive structures, which generate effects of a completely differentkind, for example motion effects, flips, image changes and so forth. Itis also possible for the free-form elements or other diffractivestructures to be combined with a thin-layer sequence, special layers(for example semiconductors) or with special colours, for exampleiridescing colours, in order in that way to achieve quite particularcolour (change) effects. In that respect it is also possible for examplefor the free-form elements according to the invention to be combined orinterleaved with other optically effective structures, for example inaccordance with EP patent No 0 375 833 B1 or for a plurality offree-form surfaces to be combined together or interleaved with eachother, so that, from the point of view of a viewer, the or a givenlens-like free-form element or one or more other optically effectivestructures appear alternately, depending on the angle at which thecorresponding substrate is viewed. A combination of the opticallyvariable elements according to the invention with print elements, mattstructures or specular surfaces is also possible.

Particularly attractive design configurations for the optically variableelements according to the invention can be achieved when the interface 3forming the effective structure is provided only region-wise with areflection-enhancing layer, in particular a metallisation, in which casefor example demetallisation can be provided here in registerrelationship with the free-form elements. For example, in theembodiments of FIGS. 3 a to 9 a, it would be possible to provide in eachcase only the free-form element, that is to say the drop-shapedfree-form surface 11 (in-FIGS. 3 a, 4 a and 5 a), the ring element 12(in FIGS. 6 a and 7 a) or the L-shaped element (in FIGS. 8 a and 9 a)with a metallisation in the region of the interface 3, but not thesurrounding interface between the layers 1 and 2. The weave-like,optically variable element of FIG. 10 could also be of a moreinteresting configuration by virtue of partial metallisation, in whichcase for example only the surface regions of the interface 3 which formthe threads 14, 15 could be metallised while there is no metallisationin the intermediate spaces between the threads 14, 15 so that in thatrespect the optically variable element would be transparent.

It should be mentioned that the interface 3 does not necessarily have tobe delimited on both sides by a lacquer or adhesive layer. Particularlywhen using the optically variable element according to the invention ina transmission mode, the interface 3 could also adjoin air, whereby therefractive index difference, which is required in the region of theinterface 3, in respect of the layers on both sides of the interface 3,could possibly be achieved in a simple fashion. Configurations of thiskind are very suitable for example for packaging or wrapping films whichare not fixed on a substrate.

Finally, precisely because it is relatively flat, an optically variableelement can also be used in combination with printed elements, forexample overprinted in a region-wise fashion.

1. An optically variable element which at least in surface portions hasan interface which forms an optically effective structure whichspatially projects and/or is set back with respect to a (notional)reference surface, wherein the optically effective structure has atleast one free-form surface appearing three-dimensionally for a viewerin the form of an alphanumeric character, a geometrical figure oranother object, wherein the free-form surface is formed by a partialregion of the interface, which is of a lens-like configuration and whichproduces a magnification, reduction or distortion effect and which formsa free-form element.
 2. An optically variable element according to claim1, wherein the interface is embedded between two layers of a layercomposite.
 3. An optically variable element according to or claim 2,wherein at least one of the layers enclosing the interface is coloured.4. An optically variable element according to claim 1, wherein thefree-form surface is in the form of a diffractive free-form element witha grating structure whose grating depth is at most 10 μm and which hasgrating lines substantially following the contour lines of the free-formsurface, wherein the spacing of the grating lines from the centralregion of the free-form surface towards the edge thereof continuouslychanges.
 5. An optically variable element according to claim 4, whereinthe grating structure of the free-form element is of such a nature thatthe respective one flanks of the grating grooves thereof extend parallelto each other and substantially parallel to a normal to the referencesurface while the angle of the respective other flanks of the gratinggrooves with respect to the normal to the reference surface changessubstantially continuously in a direction transversely with respect tothe grating lines from one grating groove to another grating groove. 6.An optically variable element according to claim 5, wherein the flanksof the grating grooves, which extend at an angle relative to the normalto the reference surface, are of a stepped configuration, wherein theflanks are approximated in respect of their optical effect by thesurfaces forming the steps.
 7. An optically variable element accordingto claim 4, wherein the grating structure of the free-form element is abinary structure which has grating grooves and grating lands which areof substantially rectangular cross-section.
 8. An optically variableelement according to claim 7, wherein the depth of the grating groovesof the grating structure of the free-form element is approximately equalover the entire free-form surface.
 9. An optically variable elementaccording to claim 1, wherein the free-form surface is formed by aholographically produced free-form element.
 10. An optically variableelement according to claim 1, wherein the free-form surface is part ofan optically effective overall structure arrangement which besides thefree-form element includes partial regions with optically variableelements producing different optical effects for the viewer.
 11. Anoptically variable element according to claim 1, wherein the opticallyeffective structure is completely or region-wise combined with athin-layer arrangement.
 12. An optically variable element according toclaim 1, wherein the interface forming the optically effective structureis provided at least region-wise with a reflection-enhancing coating.13. An optically variable element according to claim 12, wherein thereflection-enhancing coating is formed by a metal layer.
 14. Anoptically variable element according to claim 12, wherein thereflection-enhancing coating is provided in register relationship withthe at least one free-form element.
 15. An optically variable elementaccording to claim 13, wherein the register relationship is produced byregion-wise demetallisation of the interface.
 16. Use of an opticallyvariable element according to claim 1 as a security element to preventforgery of value-bearing documents or for articles to be safeguarded.17. Use according to claim 16, wherein optically variable element isincorporated into a decorative layer arrangement, which can betransferred on to a substrate, of a transfer film, in particular a hotstamping film.
 18. Use according to claim 16, wherein the opticallyvariable element is incorporated into a decorative layer arrangement ofa laminating film.